Field emitter array with enhanced performance

ABSTRACT

A microwave modulable field-effect cathode which includes at least one array of emissive tips. A microwave modulation signal is produced by a device including a microwave-controllable semiconductor modulation element which is close to the tip array, and a short microline for conveying the modulation signal to the tip array. The microline provides impedance matching between the tip array and the semiconductor modulation element. Such a device can find application as a compact field-effect cathode, as one example.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to field-effect cathodes (also calledfield emission arrays, i.e. FEAs). These cathodes have been already usedin certain types of experimental high-power electron tubes, such asrelativistic magnetrons, vircators, etc., but also in new tubes of moreconventional types, such as travelling wave tubes for applications inradar or in telecommunications.

BACKGROUND OF THE INVENTION

In this second case, the cathode is formed from at least one array oftips, comprising a substrate covered by a dielectric layer withcavities, each cavity receiving a protruding emissive tip, and a gridplaced on the surface of the dielectric layer at least partiallysurrounds the cavities.

To extract electrons from the tips, a potential difference is appliedbetween the grid and the tips. The electron emission may be modulated interms of density by modulating the voltage applied to the grid.

From an electrical standpoint, the grid and the substrate/tips assembly,which are separated by the dielectric layer, are equivalent to acapacitance of the order of 10 to 100 pF/mm² and the correspondingconductance is of the order of a few tens of mS/mm² at around 10 GHz.

Typically, if about 80 V is applied between the grid and thesubstrate/tips assembly, it is possible to extract a current of 1μA/tip, the tips having a density of the order of 10⁶ to 10⁷ per squarecentimetre.

At frequencies from 10 to 100 kHz, the impedance presented by the gridto the modulator which feeds it is essentially real and remains about afew tens of ohms. This allows a modulator of reasonable power to beused.

Developments underway relate to the operation of these microwavefield-effect cathodes. The advantage of an electron tube using such amicrowave-modulated cathode is that it can be very compact, it can beconstructed without the need for a focussing device and it has a highefficiency. It may be expected to obtain tubes whose operating principlewill be comparable to that of IOTs (Inductive Output Tubes), but whichoperate at much higher frequencies.

However, if the grid is microwave-modulated, the impedance presented bythe grid to the modulator which feeds it becomes very low because thecapacitor has a very low reactance (0.1 to 1 Ω/mm² at around 10 GHz, forexample). This requires a modulator with a bandwidth equivalent to thatof conventional tubes, with a very high power, in order to obtain asatisfactory current intensity.

The modulator is connected to the grid via a microwave transmissionline, generally a microstrip line. Another reason for requiring themodulator to have a high power is that the modulation signal applied tothe gate is reflected at the transition between the transmission lineand the grid.

In this regard, FIG. 1a shows, seen from above, a field-effect cathodeof known type. The cathode 1 comprises four sector-shaped tip arrays 2,grouped together on the same electrically conducting support 50. Eacharray comprises a conducting substrate denoted 3, a dielectric layerdenoted 4 with cavities 5 in which emissive tips 6 are placed, thedielectric layer being surmounted by a grid 7. Reference may also bemade to FIG. 1b.

The electrical power for each of the arrays 2 is supplied by means ofmicrostrip lines 8 each connecting a tip array 2 to a power modulator Mlocated some distance away. The diagram in FIG. 1a shows one modulator Mfor each tip array 2, but only one modulator may suffice for all ofthem. The microstrip lines 8 are long and occupy a much larger area thanthe tip arrays 2. The modulator M cannot be brought very close to thetip arrays 2, as it is much bulkier than the tip arrays.

In the configuration described and illustrated, the conducting support50 serves as a conducting plane for the microwave lines 8. Theinsulation of the microstrip lines is denoted 8.2 and the conductingstrip is denoted 8.3.

Each microstrip line 8 is electrically connected to a tip array 2 via aconductor 9 attached on one side to the conducting strip 8.3 and on theother side to the grid 7 of the tip array 2.

The modulators M must generate a high-level microwave signal, especiallybecause, since they are located quite far from the tip arrays 2, theyare connected thereto via lines which cause a strong reflection on thegrid side and because reflections also occur in the tip arrays 2 owingto the presence of the tips 6.

The further away from the microstrip line 8, penetrating the tip array2, the weaker the signal and the lower the current density produced bythe tips. This results in an inhomogeneous electron beam, prejudicial toproper operation of an electron tube. The modulation signal becomesineffective beyond 100 micrometres' propagation into the tip array 2.

The sector shape given to the tip arrays 2 makes it possible, if a widthof 50 to 100 micrometres is not exceeded, to improve the homogeneity ofthe beam. However, the current density is limited as it is not possibleto get close to a large number of tip arrays without considerablyincreasing the area occupied because of the space taken up by themicrostrip lines coming from the modulator M.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a cathode not having thesedrawbacks. The present invention provides a microwave-modulable,field-effect cathode, formed from at least one array of emissive tips,which is capable of emitting electrons with a current density muchhigher than that of the existing field-effect cathodes. This cathode hasthe advantage of requiring neither a conventional power modulator forcontrolling electron emission nor a high-level transmission line.Conventional modulators are expensive, greedy in terms of electricityand pose cooling problems. Transmission lines pose problems ofdifferential phase lags in the microwave signal and attenuationproblems.

To achieve this, the present invention is a microwave-modulablefield-effect cathode, comprising at least one emissive tip array, meansfor producing a microwave modulation signal and means for conveying themodulation signal to the tip array, characterized in that the means forproducing the modulation signal comprise a microwave-controllablesemiconductor modulation element which is situated very close to the tiparray, the means for conveying the modulation signal to the tip arraybeing a short microline which introduces practically negligibleperturbation and achieves impedance matching between the tip array andthe semiconductor modulation element.

The microline is especially a line of the microstrip or coplanar type,the conducting strip of which is connected at one of its ends to the tiparray and at he other end to the semiconductor modulation element.

The semiconductor modulation element is of the transistor, especiallyMESFET, type or of the diode type.

To achieve impedance matching, the conducting strip of the microline maybe configured divided into two lengths joined together by a capacitor.

The microline may also have a bias function and be connected to a biasvoltage source.

At least one element from among the tip array, the semiconductormodulation element and the microline is a discrete component.

At least two elements from among the tip array, the semiconductormodulation element and the microline are attached to the sameelectrically insulating or semi-insulating support. The two elements maybe mounted on one side of the support, the other side of which is coatedwith a conducting layer which serves as an earth plane.

It is possible to connect the microline to the tip array and/or to thesemiconductor modulation element via a wire link.

However, to prevent emission perturbations, it is advantageous to avoidwire links into the tip array. The tip array comprises an electricallyinsulating or semi-insulating substrate with, on one side, a conductingor semiconducting layer, emissive tips in electrical contact with theconducting or semiconducting layer, a dielectric layer provided withcavities, each housing one of the tips, the dielectric layer beingsurmounted by a conducting grid which at least partially surrounds thecavities. Passing through the substrate is at least one plated-throughhole which is used to electrically connect the tips to the other side ofthe substrate. The plated-through hole may be extended by a contactwhich is attached to a suitable conducting contact pad on the support.

Passing through the substrate and the dielectric layer may also be atleast one plated-through hole which is used to electrically connect thegate to the other side of the substrate. Wire links associated with thetips and/or the gate can therefore be eliminated.

To eliminate one or more wire links into the semiconductor modulationelement, it is possible to use a component compatible with a flip-chiptechnique.

The microline can be easily produced in a form integrated into theelectrically insulating or semi-insulating support, even if the tiparray and/or the semiconductor modulation element are discretecomponents.

To obtain a compact and relatively inexpensive tip-effect cathode, it isbeneficial for the tip array, the microline and the semiconductormodulation element to be integrated on the same semiconductor substrate.Preferably, the semiconductor employed is a semi-insulator such assilicon carbide.

The microline may therefore have a strip which is extended on one sideto form a grid for the tip array and on the other side to form a contactfor the semiconductor modulation element.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood and furtheradvantages will become apparent from the description which follows andfrom the appended figures which show:

FIGS. 1a, 1 b, already described, a plan view and a partial sectionalview of a known field-effect cathode;

FIG. 2, a plan view of one embodiment of a field-effect cathodeaccording to the invention;

FIGS. 3a, 3 b, embodiments of field-effect cathodes according to theinvention in which the semiconductor modulation element is a transistoror a diode;

FIGS. 4a to 4 e, various steps for producing the tip array of afield-effect cathode according to the invention;

FIGS. 5a to 5 h, various steps for producing the semiconductormodulation element of a field-effect cathode according to the invention;

FIGS. 6a, 6 b, examples of circuit diagrams for mounting field-effectcathodes according to the invention;

FIGS. 7a to 7 d, further examples of cathodes according to the inventionin which certain wire links have been eliminated; and

FIG. 8, an example of a monolithic field-effect cathode according to theinvention.

DETAILED DESCRIPTIONS OF THE INVENTION

The various components of the cathodes according to the invention havenot been shown to scale for the sake of clarity.

FIG. 2 shows schematically, seen from above, a microwave modulablefield-effect cathode according to the invention.

The cathode comprises at least one tip array A, which is itselfconventional, means S for producing a microwave modulation signal whichcontrols the electron emission and means L for conveying the signal tothe tip array A.

According to the invention, the means S for producing the microwavemodulation signal comprise a semiconductor modulation element placedjust beside the tip array A, while the means for conveying the signal tothe tip array A are a short microline introducing a practicallynegligible perturbation. The microline merely has the role ofelectrically linking the tip array to the semiconductor modulationelement. It also has the function of providing impedance matchingbetween the tip array and the semiconductor modulation element.Moreover, it can also convey at least one bias voltage.

In this way, it is possible to dispense with a bulky and expensiveconventional power modulator and with a high-level line, which modulatorand line would pose problems. Such a semiconductor modulation element Scan control the emission from several tip arrays A.

The arrangement of the tip arrays A offers a very large number ofoptions. It is possible to concentrate on a small surface area a largenumber of tip arrays A, thereby allowing higher current densities to beobtained. Each tip array A may have optimum dimensions so that there isno or very little perturbation of the modulation signal in the tip arrayA, thereby making it possible to obtain electron beams which are muchmore homogeneous than in the past. Typical dimensions for such a tiparray A are around 50 micrometres by 300 micrometres. Propagation over adistance of about 50 micrometres produces no appreciable perturbation ataround 10 GHz.

The semiconductor modulation element S which delivers the microwavemodulation signal may, for example, be a transistor or a diode. In thecase of a MESFET transistor, its area is about 500 micrometres by 200micrometres, with a substantially smaller active part AP of about 50micrometres by 200 micrometres. The microline L itself may have a lengthof approximately 100 micrometres, or even several hundreds ofmicrometres, without introducing appreciable perturbation.

FIG. 3a shows in section, an example of a field-effect cathode accordingto the invention. In this configuration, the tip array A, the microlineL and the semiconductor modulation element S are discrete components andfastened to the same dielectric support 100. In this example, the tiparray A, the microline L and the semiconductor modulation element S areeach attached by soldering to a conducting contact pad 10A, 10L, and 10Srespectively, carried on one side of the dielectric support 100. Thesolder is shown as a darkened thick line. This dielectric support 100plays an essentially mechanical role, but it may be beneficial to placeon its other main side a conductive coating 101 so as to produce a localearth plane.

In the example described, it is assumed that the microline L is amicrostrip line. It is conceivable for this to be a coplanar line and inthe cross-sectional figure it would have the same outline. Themicrostrip line L conventionally comprises a conducting plane 10.1 orearth plane, then an electrically insulating or semi-insulating layer 12followed by a conducting strip 11. A conducting plane 10.1 is attachedto the conducting pad 10L of the dielectric support 100. The conductingplane 10.1 and the conducting strip 11 may be made of nickel or an alloybased on titanium, gold, platinum, for example. The electricallyinsulating or semi-insulating layer 12 may be made of a ceramic, silicaor even silicon carbide for example.

As will be seen later in FIGS. 6a and 6 b, the strip 11 of the microlineL may be discontinuous and formed from two lengths joined together by acapacitor C, for example attached between the two lengths. Thiscapacitor C is used for the impedance matching.

The tip array A comprises an electrically insulating or semi-insulatingsubstrate 13 with, on one side, a conducting or semiconducting layer13.1, emissive tips ET in electrical contact with the conducting orsemiconducting layer 13.1, and a dielectric layer 14 provided withcavities 15, each housing one of the tips ET, the dielectric layer 14being surmounted by a conducting grid G which at least partiallysurrounds the cavities 15. The other side of the substrate 13 is coatedwith a conductive coating 10.2 in order for it to be soldered to thedielectric support 100.

When the substrate 13 is insulating, it may for example be made ofglass, alumina or silica and when it is semi-insulating it may be made,for example, of silicon carbide SiC. The materials of the substrate 13are chosen for their ability to withstand high voltages, for example ofthe order of a few hundred volts, without any damage, and hightemperatures, for example of about 400° C., these temperatures beingreached when the cathode is fitted into a microwave tube which is putinto a vacuum oven in order to obtain a high vacuum. In general, all thematerials involved in the composition of the cathode according to theinvention must be able to withstand the oven treatment and must notoutgas under vacuum.

The dielectric layer 14 may be made of silica SiO₂ for example and thegrid G and the tips ET made of molybdenum for example.

The semiconductor modulation element S in the example in FIG. 3a is atransistor. More specifically, in this example it is a MESFET-typetransistor, but of course, other types of transistors can be used. Itcomprises a conducting layer 10.3 for the soldering, then a substrate 16made of a semiconductor material having semi-insulating properties, thenan n-type semiconductor coating 18, preferably produced as two layers18.1, 18.2, the contact layer or surface layer 18.1 being n+-doped andtherefore more conducting than the n-doped active layer or base layer18.2, then two ohmic contacts, a drain D_(s) and a source S_(s), and aSchottky gate contact G_(s) between the ohmic contacts D_(s) and S_(s).Also shown in this example is a passivation layer 21 on the coating 18,it being possible for this layer to be made of silica for example.

The microline L is connected at one of its ends to the semiconductormodulation element S in the example shown at its drain D_(s), and at itsother end to the tip array A, in the example at the grid G. The grid Gof the tip array A is raised to a bias voltage E1 and the tips ET are atearth potential. The source S_(s) of the semiconductor modulator elementS is connected to earth potential and the gate G_(s) receives amicrowave modulation signal MW which the semiconductor elementamplifies. The links described above may be produced by wire bondingusing wires 20.1, for example gold wires.

In FIG. 3b, the semiconductor modulation element S is now a diode, whichmay, for example, be of the Gunn or IMPATT type. It comprises a firstconducting layer CA, which forms the cathode and will be soldered to theappropriate conducting pad 10S on the dielectric support 100. Its anodeAN is formed by a second conducting layer and these two conductinglayers AN and CA are separated by a semiconducting layer 30. Its cathodeCA is connected to earth and its anode AN is connected to one end of themicroline L.

With regard to the tip array A, FIG. 3b differs from FIG. 3a by the factthat the electrically conducting or semiconducting layer 13.1 isobtained by surface-doping the semi-insulating layer 13 thus made of asemiconductor material having semi-insulating properties, for example,silicon carbide. In the example, the tips ET are also made of thesemiconductor material having semi-insulating properties which isrendered semiconducting by doping. The tips ET could, of course, be madeof an electrically conducting material, such as molybdenum.

As regards the microline L, this is now integrated into the dielectricsupport 100. Its strip 11 is a conducting pad carried by the dielectricsupport 100, on that side to which the tip array A and the semiconductormodulation element S are attached. Its earth plane is formed by theconducting layer 101. One function of the microstrip is to act as ascreen preventing radiation leakage. FIG. 3b also shows the strip 11divided into two lengths and the capacitor C.

The techniques used to produce the tip array A may be thoseconventionally used in the semiconductor industry. An illustrativeexample is shown in FIGS. 4a to 4 e. These figures illustrate the caseof a discrete tip array like that shown in FIG. 3a.

The process starts with an electrically insulating or semi-insulatingsubstrate 13. In the example, this is assumed to be made of glass forexample. An electrically conducting layer 13.1, for example made ofmolybdenum, is deposited on top of the glass by vacuum evaporation.Next, the dielectric layer 14, which may, for example, be made ofsilica, is deposited (FIG. 4a).

The grid G, for example made of molybdenum, is then deposited (FIG. 4b).After a masking operation, for example using lithography, the conductinggrid layer G is etched, by chemical etching or by reactive ion etching(RIE), in order to form apertures 17, and then the dielectric layer 14in order to form the cavities 15 (FIG. 4c). The apertures 17 run intothe cavities 15.

The tips ET, for example made of molybdenum Mo, may be deposited byvacuum evaporation (FIG. 4d).

Next, everything lying above the grid G (FIG. 4e), that is to say theresin 25 which serves for the masking operation and the excess metalfrom the tips ET which lies on the resin 25 and is labelled 26, areremoved by chemical etching. The substrate 13, on its opposite side fromthat carrying the tips ET, is metallized at 10.2 in order to be able toattach the tip array A to the dielectric support 100 by soldering, forexample using a gold solder. This step could be carried out beforehand.

The transistor may be produced in a known manner. An illustrativeexample is shown in FIGS. 5a to 5 h and the transistor obtainedcorresponds to that illustrated in FIG. 3a.

Deposited on a substrate 16 made of a semiconductor material havingsemi-insulating properties (for example, silicon carbide) is a moreconductive coating 18 (FIG. 5a). It is preferable to produce thiscoating 18 as two layers 18.1 and 18.2, the n⁺-doped surface layer 18.1being the contact layer and the layer 18.2 between the substrate 16 andthe contact layer 18.1 being the n-doped active layer. These layers, forexample made of silicon carbide SiC or gallium nitride GaN, may bedeposited by epitaxy, whether it be liquid phase epitaxy (LPE), vapourphase epitaxy (VPE) or molecular beam epitaxy (MBE), or else by ionimplantation.

By reactive ion etching right down to the substrate 16, a plateau ormesa 19 is formed (FIG. 5b).

A trench 20 is produced in the contact layer 18.1 in a central region ofthe mesa 19 by reactive ion etching (FIG. 5c).

A passivation layer 21 is then usually deposited (FIG. 5d). This may bemade of silica SiO₂ or silicon nitride Si₃N₄, for example.

The ohmic contacts D_(s) and S_(s) are deposited after an operation ofetching the passivation layer 21 down to the contact layer 18.1, thisoperation being preceded by a masking operation, for example usinglithography (FIG. 5e). The two ohmic contacts D_(s) and S_(s), which aresubstantially identical, are then deposited, preferably at the sametime, by sputtering or evaporation in the etched locations. They areusually made of nickel. Next, the resin 25 used in the masking operationis removed (FIG. 5f).

The Schottky contact G_(s) is deposited separately, followed by carryingout, in the trench 20, an operation of etching the passivation layer 21down to the active layer 18.2, this operation being preceded by amasking operation using, for example, lithography (FIG. 5g). TheSchottky contact G_(s), for example made of titanium, is deposited bysputtering or evaporation in the etched location and then the resin 27used in the masking operation is removed. The next operation is toproduce a metallization layer (labelled 10.3) on the opposite side ofthe substrate 16 from that carrying the contacts, in order to be able tofasten the semiconductor modulation element to the dielectric support100 by soldering, for example using a gold solder (FIG. 5h).

In the foregoing description, the electrical links to the tip array Aand to the semiconductor modulation element S are in the form of wires20.1. It may be advantageous to reduce the number of wire links, or evento eliminate them.

To do this, it is possible to use a semiconductor modulation elementcompatible with a mounting technique known by the name of flip-chip.With regard to the tip array A, this may also be compatible with thistype of mounting technique. FIGS. 7a, 7 b illustrate this configuration.In the tip array A, a wire link may have a perturbing effect on theelectron emission pattern. A wire link is equivalent to a parasiticinductance.

From the standpoint of the useful area of the dielectric support 100, itis possible to reduce it by eliminating certain wire links since certainconducting pads, for example, the local earth pad in the case of the ET,may also be eliminated. This reduction in area is advantageous.

The semiconductor modulation element S shown schematically is of thetransistor type. It comprises three studs: a drain stud C_(d), a sourcestud C_(s) and a gate stud C_(g) each coming into electrical contactwith an appropriate conducting contact pad on the dielectric support100. More particularly the drain stud C_(d) comes into contact with thestrip 11 of the microline L, the gate stud C_(g) comes into electricalcontact with a conducting pad 70 via which the modulation signal to beamplified is conveyed, while the source stud C_(s) comes into contactwith a conducting pad 71 connected to the local earth via aplated-through hole 72 which, for example, passes through the dielectricsupport 100. The studs C_(d), C_(g), and C_(s) also play a mechanicalrole in holding the semiconductor modulation element S in place on thedielectric support 100, the mechanical link possibly being formed byfusion between the studs and the conducting pads.

The tip array A will now be described in greater detail, in which acontact 74 of the tips ET is formed at the base of an array on theopposite side from the tips. This tip array A could be usedindependently of the semiconductor modulation element S and of themicroline L. In the example shown in FIG. 7a, the electricallyinsulating or semi-insulating substrate 13 is pierced right through byat least one hole 73. This hole runs into the electrically conducting orsemi-conducting layer 13.1 supporting at least one tip ET. It is plumbwith a tip ET.

FIG. 7a again shows the dielectric layer 14 which includes the cavities15 and the grid G, without any modification with respect to what isshown in FIG. 3a.

This hole 73 is plated internally with metal and is extended on theopposite side from the tips ET by a contact 74 in the form of aconducting stud 740. It is this stud 740 which contributes to theelectrical connection of the tips ET and to the mechanical fastening ofthe tip array A to the dielectric support 100. This stud 740 is inelectrical contact with a conducting pad 75 carried by the dielectricsupport 100, this conducting pad 75 being connected in the example tothe local earth by any suitable means.

As illustrated in FIG. 7b, it is possible to envisage the tip contact 74not in the form of a conducting stud. In this new embodiment, the hole73 is metallized on its walls, this metallization 78 forming an end wallon the same side as the tips ET and emerging on the opposite side fromthe tips, forming a rim 741 which comes into electrical and mechanicalcontact with an appropriate conducting pad 75 on the dielectric support100. This connection may be made by soldering. In the example, thisconducting pad 75 is connected to the local earth via a plated-throughhole 76 which passes through the dielectric support 100 as far as thelocal earth plane 101. The metallization 76 is not hatched in order notto clutter up the figure.

In the example shown in FIG. 7b there are as many holes 73 as there aretips ET and the electrically conducting or semiconducting layer 13.1supporting the tips ET is discontinuous, taking the form of discs eachserving as base for a tip ET. In the examples shown in FIGS. 3 and 7a,the layer 13.1 is continuous, forming a carpet under the tips.

The holes 73 must have a relatively small diameter if the tip density inthe array is high. The order of magnitude of their diameter is less thanone micrometre. Production of these holes is tricky. To avoid producingholes which are too fine, the electrically conducting or semiconductinglayer 13.1, which in this embodiment is continuous from one tip toanother, may be extended by a region 77 containing no tip ET. Thisembodiment is illustrated in FIG. 7c. One or more holes 79 are thereforedrilled through the electrically insulating or semi-insulating substrate13 and these holes may be less fine than those plumb with the tips ET.

The metallization 80 of the holes is similar to that which has just beendescribed in the case of FIG. 7a or 7 b and the contact 74 for the tipson the opposite side from the tips takes the form either of a stud or arim. The electrical connection of the tip contact 74 may be similar tothat shown in FIGS. 7a, 7 b. The tip array may be mechanically connectedto the dielectric support 100 as in the examples shown in FIG. 3.

Another very important advantage of bringing a tip ET contact at thebase of the tip array A through the electrically insulating orsemi-insulating substrate 13 is that the thickness of insulatingmaterial between the grid G and this tip contact is considerablyincreased. Consequently, the grid-tip capacitance is significantlyreduced. In FIG. 3a, the thickness to be taken into consideration isthat of the dielectric layer 14 comprising the cavities 15, whereas inFIG. 7a it is that of the dielectric layer 14 comprising the cavities 15and that of the electrically insulating or semi-insulating substrate 13.The orders of magnitude of the thicknesses are as follows: approximatelyone micrometre in the case of the dielectric layer 14 comprising thecavities 15 and approximately 300 micrometres in the case of theelectrically insulating or semi-insulating substrate 13. The energyneeded to charge the grid-tip capacitance may be reduced for the sameelectron emission.

It may also be advantageous to eliminate the wire links from the grid Gand to make a grid contact 81 at the base of the tip array on theopposite side from the grid. FIG. 7d shows this configuration. One ormore holes 82 have been produced from the grid down to the base of thetip array through, on the one hand, the dielectric layer 14 with thecavities 15 and, on the other hand, the electrically insulating orsemi-insulating substrate 13. These holes are metallized and matters aretaken to ensure that the metallization 83 is not in electrical contactwith the electrically conducting or semiconducting layer 13.1 supportingthe tips ET, which layer may therefore be discontinuous and again formdiscs, as in FIG. 7b.

At the base of the tip array A, the metallization 83 terminates in thecontact 81 in the form of a stud or a rim, both embodiments being shownin FIG. 7d.

In the example shown in FIG. 7d, one of the contacts 81 (the one in theform of a stud) comes into mechanical and electrical contact with thestrip 11 of the microline L and the other contact (that in the form of arim) comes into mechanical and electrical contact with a conducting pad84 carried by the dielectric support 100 and connected to the biasvoltage source E1.

From the implementation standpoint, the holes may be obtained by RIE.The electrically conducting layer 13.1 and/or the gate G may be made ofnickel, which is not attacked by the etching if the latter is carriedout after the dielectric layer 14 and the grid G have been deposited.The holes may be metallized in several layers, based on titanium, nickelor gold for example. The studs and the rims may also be made of thesematerials.

Referring again to FIG. 3a, the microline L does not merely electricallyconnect the semiconductor modulation element S to the tip array A. Italso has a matching function, since the semiconductor modulation elementS and the tip array A generally have very different output impedences.The impedance of the semiconductor element may be of the order of a fewohms to a few tens of ohms, whereas that of the tip array is of theorder of an ohm or a tenth of an ohm.

The strip 11 of the microstrip line has a geometry which is suitable forfulfilling this function of matching between the tip array A and thesemiconductor modulation element S. The thickness of the insulatingsubstrate 12 contributes to this matching function.

Measures are taken for the thickness of the semiconductor modulationelement S to be of the order of or slightly greater than that of the tiparray A in order not to prevent extraction of the electrons or todeflect their paths. A maximum difference of the order of tenmicrometres is acceptable.

During operation of the cathode, the voltages to be applied to the gridG of the tip array may be such that the microline L and possibly the tiparray A are connected to bias voltage sources. FIG. 6a shows, seen fromabove, a cathode according to the invention. The semiconductormodulation element S is always a MESFET transistor. Its source S_(s)earthed, its gate G_(s) connected to a bias voltage source E3 receivesthe microwave modulation signal MW and its drain D_(s) is connected to afirst end of the microline L, which is shown as a microstrip line.

The second end of the microline L is connected to the grid G of the tiparray A. The geometry of the strip of the line L, divided into twolengths 11.1, 11.2 joined together by a capacitor C, allows matchingbetween the transistor S and the tip array A. The microstrip line L isconnected to a bias voltage source E2 on the same side as its first end.This bias is applied to the drain D_(s) of the transistor S. The wirelinks at both ends of the microline L are denoted by 20.1.

The tips ET of the tip array are connected to earth. This connection ismade by an extension of the electrically conducting or semiconductinglayer 13.1, without the covering by a dielectric layer, this extensionbeing shown in FIG. 7c.

In the example shown, the grid G of the tip array A is connected to abias voltage source E1.

Decoupling means C′, L1, L2 and L3 have been inserted in a mannercompletely conventional for those skilled in the art. For this purpose,there is a capacitor C′ between the gate G_(s) of the transistor S andthe input of the microwave modulation signal MW, an inductor L3 betweenthe bias voltage source E3 and the gate G_(s) of the transistor S, aninductor L2 between the bias voltage source E2 and the microstrip line L(on the drain D_(s) side of the transistor S), and an inductor L1between the bias voltage source E1 and the grid G of the tip array A.

Likewise, FIG. 6b illustrates a cathode according to the invention, inwhich the semiconductor modulation element S is a diode. The onlydifferences from the diagram in FIG. 6a relate to the diode connections.The cathode CA of the diode is connected to earth and the anode AN isconnected to the first end of the microline L, which is also connectedto the bias voltage source E2. A synchronization signal SY may beinjected onto the anode AN of the diode. The synchronization signal SYmay be electrical and a decoupling capacitor C″ is then placed betweenthe anode AN and the input of the synchronization signal SY. Thesynchronization signal could be optical and in this case thesemiconductor modulation element S would be an optical component, suchas a photodiode.

Instead of the cathode according to the invention comprising a tip arrayA and a semiconductor modulation element S in the form of discretecomponents, it is possible for the cathode according to the invention tobe monolithic.

FIG. 8 shows such a monolithic cathode. The electrical connections tothe local earth or to a bias voltage source have not been shown for thesake of clarity, but they may be produced using one of the methodsdescribed above. The tip array A, the microline L and the semiconductormodulation element S are integrated on the same semiconductor substrate200 made of a material having semi-insulating properties such as, forexample, silicon carbide. From the heat dissipation standpoint, this isentirely satisfactory.

This common substrate 200 has one of its main sides coated with aconducting layer 201 which serves as a local earth plane. Defined on theother main side are a region I for at least one tip array A, a region IIfor the microline L and a region III for the semiconductor modulationelement S.

The semiconductor modulation element S is produced in the region III andthis production may be carried out as illustrated in FIG. 5, thesubstrate 200 then being equivalent to the substrate 16.

The tip array A is produced in the region I and this production may becarried out as illustrated in FIG. 4, the substrate 200 then beingequivalent to the electrically insulating or semi-insulating layer 13.

The microline L is produced in the region II and its structure isequivalent to that shown in FIG. 3b. The substrate 200 corresponds inpractice to that denoted by 100 in FIG. 3b.

With such a configuration, the drain of the transistor, the strip of themicroline and the grid of the tip array may then be formed during thesame step in the same material.

Likewise, the passivation layer 21 of the semiconductor modulationelement, made of a dielectric, may extend into the region II, coveringthe substrate 200, and into the region I, forming the dielectric layerwhich includes the cavities 15.

Instead of producing a tip array A comparable to that shown in FIG. 3awith wire links, it is possible for it to be comparable to one of theexamples in FIG. 7, that is to say with tips connected to the earthplane 201 via at least one plated-through hole passing through thesubstrate 200.

Such a monolithic field-effect cathode is very beneficial as it iscompact and costs less than a cathode comprising discrete components, asit uses fewer materials and it takes less time to produce it.

What is claimed is:
 1. Microwave modulable field-effect cathode,comprising at least one array of emissive tips, and means for producinga microwave modulation signal to be sent to the tips, in which cathodethe means for producing a modulation signal comprise at least onesemiconductor element placed very close to the tip array, and animpedance-matching microline is interposed between the semiconductorelement and the tip array in order to convey the modulation signal fromthe semiconductor element to the tip array.
 2. Field-effect cathodeaccording to claim 1, wherein the microline is a line comprising aconducting strip connected at one of its ends to a tip array and at theother end to the semiconductor modulation element.
 3. Field-effectcathode according to claim 1, wherein the semiconductor modulationelement is of the transistor type or of the diode type.
 4. Field-effectcathode according to claim 1, wherein the microline has a conductingstrip divided into two lengths joined together by a capacitor. 5.Field-effect cathode according to claim 1, wherein the microline isconnected to a bias voltage source.
 6. Field-effect cathode according toclaim 1, wherein the microline is connected to the semiconductormodulation element and/or to the tip array via a wire link. 7.Field-effect cathode according to claim 1, wherein at least one elementtaken from among the tip array, the semiconductor modulation element,and the microline is a discrete component.
 8. Field-effect cathodeaccording to claim 7, wherein at least two elements taken from the tiparray, the semiconductor modulation element, and the microline areattached to a same electrically insulating or semi-insulating support.9. Field-effect cathode according to claim 8, wherein the two elementsare mounted on one side of the support, and the other side of thesupport is coated with a conducting layer which serves as a groundplane.
 10. Field-effect cathode according to claim 8, wherein themicroline is integrated into the electrically insulating orsemi-insulating support.
 11. Field-effect cathode according to claim 7,wherein the semiconductor modulation element is compatible with aflip-chip technique.
 12. Field-effect cathode according to claim 7, inwhich the tip array comprises an electrically insulating orsemi-insulating substrate with, on one side, a conducting orsemiconducting layer, emissive tips in electrical contact with theconducting or semiconducting layer, a dielectric layer provided withcavities, each housing one of the tips, the dielectric layer beingsurmounted by a conducting grid which at least partially surrounds thecavities, wherein passing through the substrate is at least oneplated-through hole which is used to electrically connect the tips tothe other side of the electrically insulating or semi-insulatingsubstrate.
 13. Field-effect cathode according to claim 12, wherein theplated-through hole is extended by an electrical contact. 14.Field-effect cathode according to claim 7, in which the tip arraycomprises an electrically insulating or semi-insulating substrate with,on one side, a conducting or semiconducting layer, emissive tips inelectrical contact with the conducting or semiconducting layer, adielectric layer provided with cavities, each housing one of the tips,the dielectric layer being surmounted by a conducting grid which atleast partially surrounds the cavities, wherein passing through thesubstrate and the dielectric layer are at least one plated-through holewhich is used to electrically connect the grid to the other side of thesubstrate.
 15. Field-effect cathode according to claim 1, wherein thetip array, the microline, and the semiconductor modulation element, areintegrated on a same semiconductor substrate.
 16. Field-effect cathodeaccording to claim 15, wherein the semiconductor substrate is made of asemi-insulating material such as silicon carbide.
 17. Field-effectcathode according to claim 15, wherein the microline has a strip whichis extended on one side to form a grid for the tip array and on theother side to form a contact for the semiconductor modulation element.18. Cathode according to claim 1, wherein the microline has a length notexceeding several hundreds of micrometres.